The present invention relates to a technique for preventing information stored on a magnetic disk in a magnetic disk drive from being lost as time elapses after the write.
In a magnetic disk drive, information is recorded by magnetizing magnetic particles of a magnetic disk. However, information stored magnetically on the magnetic disk has a property that it is degraded or lost with time. In the magnetic disk, information is recorded magnetically as bits by virtue of numerous magnetic particles called grains, which are magnetized by a magnetic field of a write head and maintained in a fixed direction as shown in FIG. 1. The grains have a plurality of stable states and are oriented in a specific stable direction by magnetization. However, even once the grains are magnetized in the specific direction by the magnetic field of the magnetic write head, the grains are affected by thermal energy and begin changing the orientation into other stable directions gradually and, in turn, affect adjacent grains so that more and more grains become oriented in the directions different from the initial direction upon the magnetization. As a result, the read signal may be degraded in terms of resolution and gain. This phenomenon is known as thermal decay. Further, as shown in FIG. 1, at magnetization transition regions located in the circumferential direction of tracks, the magnetic particles in one direction are likely to be affected by the magnetic field of the magnetic particles in the other magnetic direction.
Further, as the track density is increased more and more, the magnetic particles may be affected by the magnetic field generated when a write head writes data into adjacent tracks. This phenomenon is called as adjacent track interference. Therefore, if the information written on the magnetic disk is left as it is for decades or more, it may be degraded and may not be able to be read. The noise in SN ratio in the magnetic disk mainly consists of so-called transition noise. If the magnetization transition region crosses the track straightly, the possibility to electrically distinguish the read information from that in adjacent regions is increased but, if large-sized grains are used, the shape of the magnetic transition region tends to deviate from the straight line and cause the transition noise. Therefore, it is desirable to reduce the size of grains in order to improve the SN ratio. However, the small grains are likely to transit to other stable states under the influence of small energy and, therefore, it is susceptible to effects such as the thermal decay and adjacent track interference.
Japanese Patent Laid-Open No. Hei 7-272471 discloses a technique for recording cumulative operation time of each sector on a disk and taking any action when the cumulative operation time reaches a predetermined value. Japanese Patent Laid-Open No. 2000-187926 discloses a technique for prohibiting a recording medium from being written newly, making an alarm, or making a backup according to the degree of estimated degradation by reading information about the date of manufacture of the recording medium and comparing elapsed time after the manufacture with a predetermined level. Japanese Patent Laid-Open No. Hei 04-248164 discloses a technique for measuring the degree of degradation sequentially and stopping write and read operations.